The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's or dryer fabric for use on the dryer section of a paper machine, such as on a single-run dryer section.
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry on a forming fabric in the forming section of a paper machine. A large amount of water drains from the slurry through the forming fabric, leaving the fibrous web on the surface thereof.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom. This water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web, by now a sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders which are heated from within by steam. The sheet is directed in a serpentine path sequentially around each in the series of drums by one or more dryer fabrics, which hold it closely against the surfaces of the drums. The heated drums reduce the water content of the sheet to a desirable level through evaporation.
In a dryer section, the dryer cylinders may be arranged in a top and a bottom row or tier. Those in the bottom tier are staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it passes alternately between the top and bottom tiers as it passes first around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section.
As shown in FIG. 5, in dryer sections, the top and bottom tiers of dryer cylinders may each be clothed with a separate dryer fabric 99. In such a situation, paper sheet 98 being dried passes unsupported across the space, or “pocket”, between each dryer cylinder and the next dryer cylinder on the other tier.
In a single tier dryer section, a single row of cylinders along with a number of turning cylinders or rolls may be used. The turning rolls may be solid or vented.
In order to increase production rates and to minimize disturbance to the sheet, single-run dryer sections are used to transport the sheet being dried at high speeds. In a single-run dryer section, such as that shown in FIG. 6, a paper sheet 198 is transported by use of a single dryer fabric 199 which follows a serpentine path sequentially about dryer cylinders 200 in the top and bottom tiers.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks. “Drop off” can also reduce machine efficiency if it leads to sheet breaks.
Many paper mills have addressed this problem by machining grooves into the dryer cylinders or rolls or by adding a vacuum source to those dryer rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric, although both are expensive.
The present invention provides a solution to this problem in the form of a dryer fabric having void volume on the surface which does not come into contact with the paper web, that is, on the backside surface. The void volume gives the air carried into the compression wedge somewhere to go other than through the fabric.